1. Field of the Invention
The present invention relates to a noise filter provided with an inductor which is attached to a conductor such as a power supply line or a ground line, suppresses noise induced on the conductor.
2. Description of the Related Art
A conventional noise filter comprises an inductor and a capacitor, having a function to discriminate a frequency of an undesired signal called xe2x80x9cnoisexe2x80x9d from others to thereby pass only the signal of power supply frequencies etc. to the side of equipment. The noise has a frequency of, for example, 10 kHz or higher. The commercial power supply has a frequency of 50 or 60 Hz in Japan. Note here that in this specification the term xe2x80x9cequipmentxe2x80x9d refers to an electric apparatus, an electric device, etc. in general.
FIG. 6 is a circuit diagram showing the conventional noise filter. The following will describe the noise filter with reference to FIG. 6.
A noise filter 70 shown in FIG. 6[1] is a two-terminal type noise filter for use on the ground line, comprised of one inductor 71. One terminal 72 of the two is connected to the ground and the other terminal 73, to the equipment. Also, inductance of the inductor 71 is set so that a power supply frequency current including a leakage current may be discharged from the equipment to the ground and also a noise current induced on the ground line may be blocked.
A noise filter 80 shown in FIG. 6 [2] is a four-terminal type noise filter for use on the power supply line, comprised of four inductors 81-84 and three capacitors 85-87. Respectively, one terminals 88 and 89 are connected to the power supply and the other terminals 90 and 91, to the equipment. Also, the inductance of the inductors 81-84 and the static capacitance of the capacitors 85-87 are set so that the power supply frequency current may be flown from the power supply to the equipment and the noise current induced on the power supply line may be blocked.
The conventional noise filters, however, have the following problems.
A noise power referred to as xe2x80x9cnoisexe2x80x9d is induced not only in a steady-state current on the power supply line or the ground line but also in a pulse-state current with non-periodically. In such a case, the inductors and capacitors of the noise filter, which accumulate power thereon owing to magnetic and electric fields respectively, release the accumulated power when noise power stops flowing thereto. As such, this released power may cause the equipment to fail or temporarily deteriorate in functioning.
In order to reduce the noise current induced on the ground line, an inductor such as a coil is used. In particular, large equipment connected to the ground line has a considerably large capacitance with respect to the ground, which may sometimes be combined with the inductance of the inductor to give rise to series resonance. A resultant resonance frequency current may flow into the equipment, thus generating a failure due to noise.
In view of the above, it is an object of the present invention to provide a noise filter which can solve a problem due to power released therefrom and a problem due to a resonance frequency current caused by combination of the inductance and the earth capacitance thereof.
The present inventor greatly engaged in researches to solve these problems and found that xe2x80x9ca problem due to power released from a noise filter occurs if it does not have a function to dissipate a noise current (transform it into thermal energy)xe2x80x9d. The present invention is based on this finding. That is, by connecting a resistor to the noise filter in parallel with the inductor, the noise current can be dissipated at the resistor. Also, in such a circuit configuration, the resistor will act also to attenuate a series resonance current generated by combination of the capacitance-ground and the inductance. It is detailed as follows.
The noise filter according to present invention is attached to a conductor such as a power supply line or a ground line and which is provided with an inductor for suppressing noise induced on said conductor, wherein a resistor is connected in parallel with said inductor. In this configuration, the power supply current having low frequency passes through the inductor without power loss while bypassing the resistor. On the other hand, high frequency noise current including the resonance frequency current does not pass the inductor and the high frequency noise current is consumed in the resistor. Accordingly, since an electric power caused by the noise is not accumulated in the noise filter, the problem caused by discharging of the electric power is not occurred. Also, since the resonance frequency current caused by the noise filter and the earth capacitance is consumed in the resistor, the problem caused by the resonance frequency current is not occurred.
The noise filter may be configured that a parallel circuit consisting of the inductor and the resistor is attached to one ground line, with one terminal thereof connected to the ground and the other terminal thereof connected to equipment. The noise filter of this configuration is called a ground line noise filter. The number of inductors and resistors may be single or plural. When there are a plurality of inductors, a configuration of at least one inductor being connected in parallel with the resister may be acceptable.
Assuming an angular frequency of a power supply current to be xcfx89p[rad], a lower limit angular frequency of a noise current to be xcfx89n[rad], inductance of said inductor to be L[H], and resistance of said resistor to be R[xcexa9], preferably a relationship of 10(xcfx89pxc2x7L) less than R less than (xcfx89nxc2x7L)/10 is established, and more preferably a relationship of 100(xcfx89pxc2x7L) less than R less than (xcfx89nxc2x7L)/100 is established, and most preferably a relationship of 1000(xcfx89pxc2x7L) less than R less than (xcfx89nxc2x7L)/1000 is established. By thus narrowing a range of the value of R, it is possible to obtain well balanced characteristics that an attenuation quantity at xcfx89p is small appropriately and that at xcfx89n is large appropriately.
Also, a relationship of (xcfx89nxc2x7L)/Rxe2x89xa71/(2xcfx89n) may be established. In this case, the power dissipated at the resistor exceeds the power accumulated on the inductor.
When the power supply line is provided more than one, the noise filter may be configured that the parallel circuit consisting of the inductor and the resistor is provided to each of the power supply lines and a capacitor is provided between the power supply lines. The noise filter of this configuration is called a noise filter for use on a power supply line. The number of inductors and resistors may be single or plural. When there are a plurality of inductors, a configuration of at least one inductor being connected in parallel with the resister may be acceptable.
The resistor may be a variable resistor. As the noise filter is attached to various units of equipment, they have fluctuations in value of the earth capacitance. The resultant fluctuations in resonance frequency can also be accommodated properly by changing the resistance of the variable resistors.
The noise filter may have a configuration that the inductor is a toroidal coil, the resistor is a variable resistor, the parallel circuit consisting of the toroidal coil and the variable resistor is housed in a frame, the variable resistor is arranged in a space surrounded by an inner peripheral wall of the toroidal coil, and resistance varying means for varying resistance of the variable resistor is provided at such a position as to be able to be operated from an outside of the frame. By operating the resistance varying means from the outside of the frame, fluctuations in value of the earth capacitance can be easily accommodated. Also, since the variable resistor is arranged in the space surrounded by the inner peripheral wall of the toroidal coil, the space in the frame can be utilized effectively.